Bolt Preload

Bolt preload is the tension intentionally created in a bolt when it is tightened. As you apply torque (or another tightening method), the bolt stretches slightly like a spring, and that stretch produces a clamping force that squeezes the joint members together. In many assemblies, it’s the preload, not the bolt shank in shear, that primarily keeps the joint from slipping, separating, or leaking.

Getting the right preload matters because too little preload can allow joint movement, vibration loosening, fatigue cracking, and gasket leaks, while too much preload can yield the bolt, crush joint materials, or strip threads. Preload is influenced by many factors, especially friction in the threads and under the head or nut, so the same torque can produce very different preload depending on lubrication, coatings, surface finish, and fastener condition. For higher reliability, preload can be controlled or verified using methods like torque-angle tightening, direct tension indicators (DTIs), calibrated tensioning tools, or ultrasonic measurement rather than torque alone.